Wafer rapid thickness measurement system and method based on optical fiber differential

Abstract

The present invention relates to a wafer rapid thickness measurement system based on fiber-optic differential, including a light source module, a beam splitter module, a measuring path, a reference path, a detector module and a main control module, wherein: the light source module is used to generate and output a near-infrared light beam; the measuring path is used to converge the measuring light beam and incident on a wafer sample to be measured to form a small light spot, and the reflected light reflected by the wafer to be measured is returned to the beam splitter module and then enters the detector module, and a measuring path shutter is arranged on the measuring path; the reference path is used to converge the reference light beam and incident on a reference sample to form a small light spot, and the reflected light reflected by the reference sample is returned to the beam splitter module and then enters the detector module, and a reference path shutter is arranged on the reference path; the detector module includes a near-infrared high-resolution spectrometer. The present invention also provides a wafer rapid thickness measurement method using the above system.

Description

Technical Field

[0001] The present invention relates to the field of optical measurement, and in particular to a wafer thickness measurement system and method based on optical fiber differential reflection spectroscopy. Background Art

[0002] The development and manufacturing of SOI, which is widely used in MEMS and microelectronics, faces unique challenges, among which the most important one is the measurement of silicon wafer thickness. The current optical method for measuring wafer thickness mainly uses the method of optical interference, and the range of thickness measurement mainly depends on the limitations of the wavelength of light used and the resolution of the detector. Taking silicon wafers as an example, due to the limitations of the detector, it is difficult for the current commercially available rapid thickness measuring instruments to take into account the needs of large-scale thickness measurement during the thinning process of wafers: one is to measure thin silicon wafers, using a wide-spectrum near-infrared light source and a large-range low-resolution detector; the other is to measure thick silicon wafers, using a narrow-band near-infrared light source and a small-range high-resolution detector. Summary of the invention

[0003] In view of this, the purpose of the present invention is to provide an accurate, fast, simple structure, differential-based fast thickness measurement system for a large range of micron-level wafers. The technical solution is as follows:

[0004] A wafer rapid thickness measurement system based on optical fiber differential includes a light source module, a beam splitting module, a measurement path, a reference path, a detector module and a main control module, wherein:

[0005] A light source module, used for generating and outputting a near-infrared light beam;

[0006] The measuring path is used to converge the measuring light beam and incident on the wafer sample to be measured to form a small light spot, and to make the reflected light reflected by the wafer to be measured return to the beam splitting module and then enter the detector module. A measuring path shutter is arranged on the measuring path;

[0007] A reference path is used to converge the reference light beam and incident on the reference sample to form a small light spot, and to make the reflected light reflected by the reference sample return to the beam splitting module and then enter the detector module. A reference path shutter is arranged on the reference path;

[0008] A detector module is used to collect light intensity data corresponding to the wavelength of the reflected light reflected by the wafer sample to be tested and the reflected light reflected by the reference sample; the detector module includes a near-infrared high-resolution spectrometer;

[0009] A beam splitting module, used for transmitting the light output by the light source module to the measuring path and the reference path, and transmitting the reflected light reflected by the measuring path and the reflection path to the detector module;

[0010] The main control module is used to control the light source module, detector module, and on / off of the measurement path and reference path of the measurement system, and to process and calculate the light intensity data collected by the detector module to calculate the wafer thickness.

[0011] Furthermore, the central wavelength of the light source is 1550nm and the bandwidth is 40nm; the working band of the near-infrared high-resolution spectrometer is 1510nm to 1595nm.

[0012] Furthermore, the beam splitting module 3 can use a 2X2 fiber optic splitter.

[0013] The present invention also provides a thickness measurement method implemented by the wafer rapid thickness measurement system based on optical fiber differential, comprising the following steps:

[0014] Step A: The light beam passing through the measuring focusing lens is vertically irradiated onto the wafer to be measured, and a focusing operation is performed; the light beam passing through the reference focusing lens is vertically irradiated onto a standard sample with a known reflectivity, and a focusing operation is also performed;

[0015] Step B: Determine the optical constants of the wafer to be tested, and calculate the group refractive index n of the wafer to be tested based on the central wavelength and bandwidth of the light source g ;

[0016] Step C: The main control module controls the shutters of the measurement path and the reference path to use the spectrum data collected by the detector module. For thicker wafers to be tested, the detector module only collects the reflection spectrum data of the wafer to be tested in the measurement path; for thin wafers to be tested, the detector module collects the reflection spectrum data of the wafer to be tested and the reflection spectrum data of the reference sample in a time-sharing manner;

[0017] Among them, the design thickness of the wafer to be tested is known to be h 厚 , the method for judging whether the wafer to be tested is a thicker wafer to be tested or a thinner wafer to be tested is as follows:

[0018] Assume that the minimum value of the measured wavelength of the near-infrared high-resolution spectrometer is λ1, and the maximum value is λ2, which is the maximum value of the spectrometer wavelength.

[0019]

[0020] If the wafer to be tested is thicker, the wafer to be tested is judged to be a relatively thick wafer to be tested; otherwise, the wafer to be tested is judged to be a relatively thin wafer to be tested;

[0021] Step D: The main control module characterizes the thickness of the wafer to be measured through calculation according to the reflection spectrum data collected by the detector module.

[0022] Furthermore, step C includes the following sub-steps:

[0023] Sub-step C1: For a thicker wafer to be tested, the main control module controls the shutter of the measurement path to be opened and the shutter of the reference path to be closed, so as to collect the reflection spectrum data of the thicker wafer to be tested;

[0024] Sub-step C2: For a thinner wafer to be tested, the main control module controls the shutters of the measurement path and the reference path to be opened alternately in time sharing to collect the reflection spectrum data of the thinner wafer to be tested and the reflection spectrum data of the reference sample.

[0025] Further, step D is specifically as follows:

[0026] For the collected reflectance spectrum data of the thicker wafer to be tested, the thickness is extracted by transforming it into the frequency domain using Fourier transform;

[0027] For a thinner wafer to be tested, a differential reflectivity model of the wafer to be tested is established based on the optical constants measured in step B; the differential reflection signal of the wafer to be tested is calculated based on the reflection spectrum data of the measurement path and the reference path collected by the detector module; and then the thickness of the thinner wafer to be tested is calculated by fitting the differential reflection signal of the wafer to be tested with the differential reflectivity model.

[0028] The present invention is based on a fiber-optic differential spectral reflection measurement method, with multi-mode hybrid measurement as a technical feature, which solves the difficulty of measuring silicon wafers with a large thickness range. The measurement system only needs one detector, the measurement device is simple, has a high spatial resolution, and is easy to integrate into the semiconductor equipment site. The wafer thickness measurement system and method based on fiber-optic differential reflection spectroscopy of the present invention and the method have at least one or part of the following beneficial effects:

[0029] (1) Capable of quickly measuring the thickness of a wide range of wafers;

[0030] (2) Simple device, easy to integrate

[0031] (3) Using the reference path can effectively suppress the measurement error caused by light intensity BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of a thickness measurement system based on difference according to an embodiment of the present invention;

[0033] Figure 2 The figure is a flowchart of a thickness measurement method based on difference according to an embodiment of the present invention.

[0034] In the above drawings, the meanings of the reference numerals are as follows:

[0035] 1-Near infrared light source 2-2x2 bifurcated optical fiber

[0036] 3-Detector module 4-Measurement path focusing lens

[0037] 5-Measurement path shutter 6-Test sample fixture

[0038] 7- Reference path focusing lens 8- Reference path shutter

[0039] 9- Reference sample fixture 10- Main control module DETAILED DESCRIPTION

[0040] The present invention provides a wafer thickness measurement system and method based on fiber-optic differential reflection spectroscopy, in which the light output by a light source passes through a bifurcated optical fiber, a focusing lens and a shutter to form a measurement path and a reference path respectively, thereby enabling accurate measurement of the thickness of thin and thick samples to be measured, as well as real-time measurement during the thinning process.

[0041] The wafer thickness measurement system and method based on fiber-optic differential reflection spectroscopy provided by the present invention can realize real-time measurement of wafers in the process of thinning, and the measurement thickness range is (1μm~1mm); the system includes a light source module, a detector module, a beam splitter module, a measurement path, a reference path and a main control module; wherein the light source module is used to generate and output a near-infrared light beam; the measurement path is used to converge the measurement light beam and incident on the wafer sample to form a small light spot, and make the reflected light reflected by the sample to be measured return to the measurement beam splitter module and then enter the detector module; the reference path is used to The reference light beam is converged and incident on the reference sample to form a small light spot, and the reflected light reflected by the reference sample is returned to the measurement beam splitting module and then enters the detector module; the beam splitting module is used to connect the light source module and the detector module, transmit the light output by the light source module to the measurement path and the reference path, and receive and transmit the reflected light reflected by the measurement path and the reflection path to the detection module; the main control module is used to control the on-off of the light source module, the detector module, the measurement path and the reference path of the measurement system, and process and calculate the reflection spectrum data collected by the detector module, so as to calculate the thickness of the wafer.

[0042] Figure 1 FIG. 1 is a schematic diagram of a fast wafer thickness measurement system based on differential wafer according to an embodiment of the present invention. Figure 1As shown, the light source module 1 includes: a near-infrared light source. The near-infrared light source here can be a near-infrared superluminescent diode (SLD), but it is not limited to this. It should be noted that the wavelength band of the light source must correspond to the subsequent selected detector band. Here, the central wavelength of the light source is selected to be 1550nm and the bandwidth is 40nm. The beam splitting module 3 can use a 2X2 fiber optic splitter. The detector module 2 includes: a near-infrared high-resolution spectrometer with a wavelength band of 1510nm to 1595nm and a resolution of 0.2nm. The measurement path includes: a measurement path focusing lens 4, a measurement path shutter 5, and a sample fixture 6 to be tested. The reference path includes: a reference path focusing lens 7, a reference path shutter 8, and a reference sample fixture 9, wherein the measurement path focusing lens 4 and the reference path focusing lens 7 can use the same batch of fiber optic focusing accessories, the measurement path shutter 5 and the reference path shutter 8 can use electric shutters, and the sample fixture 6 to be tested and the reference sample fixture 9 can use a sample stage with a leveling device. The main control module includes: a computer 10, and a Raspberry Pi can be used for control and data processing operations.

[0043] The spectral measurement range of this embodiment is 1510nm-1595nm, and the lateral optical resolution is better than 20μm.

[0044] The present invention also provides a method for measuring the thickness of a large-scale wafer using the above-mentioned differential wafer rapid thickness measurement system. Figure 2 FIG. 1 is a flowchart of measuring wafer thickness according to an embodiment of the present invention. Figure 2 As shown, the method comprises the following steps:

[0045] Step A: Vertically illuminate the measurement path focusing lens on the wafer to be measured, and adjust the lift-off height for focusing. Vertically illuminate the reference path focusing lens on the standard sample with known reflectivity, and also perform the focusing operation;

[0046] Step B: Determine the optical constants of the wafer to be tested: select the wafer to be tested, define the optical reflection model of the wafer sample to be tested as: air-wafer to be tested-air, use the standard dielectric function as the physical model of each material, and calculate the optical constants of the wafer to be tested based on the polarization parameters measured by the ellipsometer, and then calculate the group refractive index under the selected band;

[0047] Step C: The main control module controls the shutters of the measurement path and the reference path to use the spectrum data collected by the detector module; for thicker wafers to be tested, the detector module only collects spectrum data of the wafer to be tested in the measurement path; for thinner wafers to be tested, the detector module collects spectrum data of the wafer to be tested and spectrum data of the reference sample in a time-sharing manner;

[0048] Specifically, the standards for the thin and thick wafers mentioned in this step are defined as follows: For thick silicon wafers, Fourier transform is used to the frequency domain to extract the thickness. Therefore, it is necessary to ensure that the oscillation period of the reflection spectrum is greater than 2 in this band. Therefore, the thickness of the thick silicon wafer is h 厚 for:

[0049]

[0050] Wherein, λ1 is the minimum wavelength of the spectrometer, which is 1510 nm in the embodiment of the present invention, λ2 is the maximum wavelength of the spectrometer, which is 1595 nm in the embodiment of the present invention, and n g is the group refractive index at the central wavelength in this band, which is 3.605 in this band; calculate h 厚 ≈8μm.

[0051] Specifically, the following sub-steps C1 are included: when the thickness of the wafer to be tested is greater than 8 μm, the main control module controls the shutter of the measurement path to be opened and the shutter of the reference path to be closed, so as to collect the reflection spectrum data of the thicker wafer to be tested; sub-step C2: when the thickness of the wafer to be tested is less than 8 μm, the main control module controls the shutters of the measurement path and the reference path to be opened alternately in time-sharing, so as to collect the reflection spectrum data of the thinner wafer to be tested and the reflection spectrum data of the reference sample;

[0052] Step D: The main control module characterizes the thickness of the wafer to be measured by calculation based on the spectral data collected by the detector, including:

[0053] Sub-step D1: For a thicker wafer to be tested, there is obvious interference in the reflection spectrum data collected by the detector module. The main control module performs frequency extraction on the collected reflection spectrum to characterize the thickness of the wafer to be tested.

[0054] Sub-step D2: For a thinner wafer to be tested, a differential reflectivity model of the wafer to be tested is established according to the effective medium model based on the optical constants measured in step B, wherein the model is air layer / wafer to be tested layer / air layer. The differential reflectivity model DR is as follows:

[0055]

[0056] In the formula, R T It represents the reflectivity of the wafer to be tested, and R0 represents the reflectivity of the reference standard sample.

[0057] Sub-step D3: For a thinner wafer to be tested, the differential reflection signal of the wafer to be tested is calculated based on the two-way reflection spectrum data collected by the detector module, and then the thickness of the thinner wafer to be tested is calculated by fitting the differential reflection signal of the wafer to be tested through the differential reflectivity model of sub-step D2, specifically:

[0058] For thinner wafers to be tested, the differential reflection signal DR of the wafer to be tested is calculated based on the two reflection spectrum data collected by the detector module. measure , the formula is as follows:

[0059]

[0060] In the formula, I T represents the reflectance spectrum of the wafer to be tested, and I0 represents the reflectance spectrum of the reference standard sample;

[0061] Then, the differential reflectivity model DR of sub-step D2 is used to fit the differential reflection signal DR of the wafer to be tested. measure Calculate the thickness of the thinner wafer to be tested.

[0062] So far, the embodiments of the present invention have been described in detail in conjunction with the drawings. It should be noted that the implementation methods not shown or described in the drawings or the body of the specification are all forms known to ordinary technicians in the technical field and are not described in detail. In addition, the above definitions of various elements and methods are not limited to the various structures, methods and models of the embodiments, and ordinary technicians in the field can simply replace or substitute them.

[0063] In summary, the present invention can achieve rapid measurement of wafer thickness. The differential optical path set in the present invention can effectively reduce the common mode noise in the optical path, and significantly expand the range of thickness measurement compared to traditional single optical path measurement. In addition, the optical path is simple and easy to integrate into the semiconductor equipment site to achieve online detection.

Claims

1. A wafer rapid thickness measurement method based on fiber-optic differential, the thickness measurement system used includes a light source module, a beam splitting module, a measurement path, a reference path, a detector module and a main control module, wherein: A light source module, used for generating and outputting a near-infrared light beam; A measuring path is used to converge the measuring light beam and incident on the wafer sample to be measured to form a light spot, and to make the reflected light reflected by the wafer to be measured return to the beam splitting module and then enter the detector module. A measuring path shutter is arranged on the measuring path; A reference path is used to converge the reference light beam and incident on the reference sample to form a small light spot, and to make the reflected light reflected by the reference sample return to the beam splitting module and then enter the detector module. A reference path shutter is arranged on the reference path; A detector module is used to collect light intensity data corresponding to the wavelength of the reflected light reflected by the wafer sample to be tested and the reflected light reflected by the reference sample; the detector module includes a near-infrared high-resolution spectrometer; A beam splitting module, used for transmitting the light output by the light source module to the measuring path and the reference path, and transmitting the reflected light reflected by the measuring path and the reflection path to the detector module; The main control module is used to control the light source module, detector module, and on / off of the measurement path and reference path of the measurement system, and to process and calculate the light intensity data collected by the detector module to calculate the wafer thickness; The wafer rapid thickness measurement method comprises the following steps: Step A: The light beam passing through the measuring focusing lens is vertically irradiated onto the wafer to be measured, and a focusing operation is performed; the light beam passing through the reference focusing lens is vertically irradiated onto a standard sample with a known reflectivity, and a focusing operation is also performed; Step B: Determine the optical constants of the wafer to be tested, and calculate the group refractive index n of the wafer to be tested based on the central wavelength and bandwidth of the light source g ; Step C: The main control module controls the shutters of the measurement path and the reference path to use the spectrum data collected by the detector module. For thicker wafers to be tested, the detector module only collects the reflection spectrum data of the wafer to be tested in the measurement path; for thin wafers to be tested, the detector module collects the reflection spectrum data of the wafer to be tested and the reflection spectrum data of the reference sample in a time-sharing manner; Among them, the design thickness of the wafer to be tested is known to be h 厚 , the method for judging whether the wafer to be tested is a thicker wafer to be tested or a thinner wafer to be tested is as follows: Assume that the minimum value of the measured wavelength of the near-infrared high-resolution spectrometer is λ1, and the maximum value is λ2, which is the maximum value of the spectrometer wavelength. If the wafer to be tested is thicker, the wafer to be tested is judged to be a relatively thick wafer to be tested; otherwise, the wafer to be tested is judged to be a relatively thin wafer to be tested; Step D: The main control module characterizes the thickness of the wafer to be measured through calculation according to the reflection spectrum data collected by the detector module.

2. The method for rapid wafer thickness measurement based on optical fiber differential according to claim 1, characterized in that: The central wavelength of the light source is 1550nm and the bandwidth is 40nm; the working band of the near-infrared high-resolution spectrometer is 1510nm to 1595nm.

3. The method for rapid wafer thickness measurement based on optical fiber differential according to claim 1, characterized in that: The beam splitting module 3 uses a 2X2 fiber optic splitter.

4. The method for rapid wafer thickness measurement based on optical fiber differential according to claim 1, characterized in that: Step C includes the following sub-steps: Sub-step C1: For a thicker wafer to be tested, the main control module controls the shutter of the measurement path to be opened and the shutter of the reference path to be closed, so as to collect the reflection spectrum data of the thicker wafer to be tested; Sub-step C2: For a thinner wafer to be tested, the main control module controls the shutters of the measurement path and the reference path to be opened alternately in time sharing to collect the reflection spectrum data of the thinner wafer to be tested and the reflection spectrum data of the reference sample.

5. The method for rapid wafer thickness measurement based on optical fiber differential according to claim 4, characterized in that: Step D is specifically as follows: For the collected reflectance spectrum data of the thicker wafer to be tested, the thickness is extracted by transforming it into the frequency domain using Fourier transform; For a thinner wafer to be tested, a differential reflectivity model of the wafer to be tested is established based on the optical constants measured in step B; the differential reflection signal of the wafer to be tested is calculated based on the reflection spectrum data of the measurement path and the reference path collected by the detector module; and then the thickness of the thinner wafer to be tested is calculated by fitting the differential reflection signal of the wafer to be tested with the differential reflectivity model.

Images